A TET device is a device for providing electrical power to an implanted mechanical or electrical medical device, such as a bone growth stimulator, muscle stimulator, prosthetic heart or a ventricular assist device, without having to breach the skin to lead conducting wires therethrough.
In U.S. Pat. No. 5,350,413 John Miller discloses a TET device with a high-energy transfer efficiency. Such a device allows for efficient transfer of energy between two coils having fixed spacing. Unfortunately, as one coil is located within a body and another coil is located outside the body, maintaining coil separation at a constant distance is difficult. Changes in coil spacing result in variation of the induced voltage and, as the distance increases, the power transfer efficiency drops off rapidly.
In an article entitled "Development of an Autotuned Transcutaneous Energy Transfer System," John Miller, G. Belanger, and T. Mussivand suggest an autotuning circuit to overcome this problem. The autotuning circuit compares various voltages and currents present within a driving circuit external to the body to determine a tuning requirement. Such tuning enables the tuning of energy transfer where the coil spacing varies.
It has been found that the autotuning function disclosed addresses the problem of power coupling efficiency, but fails to address a further problem of internal voltage control. In driving implanted medical devices, energy coupling efficiency and voltage control are separate but related issues to address. Better coupling efficiency results in lower operating cost and improved battery life. Voltage control results in improved device operation and increased safety. In fact, some devices will fail from excessive applied voltage.
Further, it has been found that efficiency is affected by several factors some of which include power coupling related factors such as spacing and load related factors such as medical device load requirements or faults. Unfortunately, autotuning does not address the issue of providing additional energy when required by a medical device.
It has also been found, that prolonged exposure to electromagnetic fields results in damage to human skin. Resulting damage is not believed to be linearly related to the electromagnetic field strength and exposure time. It is believed that high-energy electromagnetic fields above a certain threshold damage human skin and adjacent tissue significantly more rapidly than low energy electromagnetic fields. Since a TET device provides energy to an implanted system and some of these implanted systems require significant power, the damage to tissue such as human skin is a significant drawback to extended use of TET devices. Reducing the electromagnetic field strength and/or reducing exposure time increases tissue longevity.
Two common approaches are known for addressing the problem of tissue damage. The first, skin grafting, is a surgical technique wherein dead tissue is replaced with healthy tissue from another area of a patient's body. Surgical techniques of this type are generally, not desirable. The second technique involves the design and implantation of lower power devices. Unfortunately, a device such as a heart pump requires significant power even when efficiently implemented.
It would be advantageous to provide a TET system that was less prone that the prior art to the problems of tissue damage.
In U.S. Pat. No. 5,350,413, John Miller further discloses an IR telemetry module for providing bidirectional communications. It is known that infra red telemetry is affected by skin pigmentation. As a transceiver disclosed by John Miller is implanted beneath a layer of skin, such considerations are important. It has been found that highly pigmented skin attenuates IR signals and renders a system as disclosed by John Miller substantially unworkable. Further, dirt and other obstructions like clothing or casings affect IR telemetry and can render it inoperable. For a television remote control, this is an acceptable limitation; for medical devices required by an individual, an inoperable TET is unacceptable.
Limitations are inherent in an IR telemetry link. IR is an optical communications means requiring an optical path between transmitter and receiver. Absent fibre or waveguides, IR telemetry is highly directional and limits a system to a single transmitter operating at a time in a direction. The directional nature of IR telemetry requires substantial alignment for optical communication.
Until recently, IR telemetry has been limited to low frequency communications. At low frequencies, it is difficult to multiplex channels, as a serial link requires higher frequencies than a true multi-channel implementation. Unfortunately, as noted above, IR telemetry is not suited to true multi-channel communications. The advent of high speed IR circuits may allow for channel multiplexing using a known technique such as time division multiplexing (TDM); however, this does not overcome previously mentioned shortcomings of IR.